The plant growth stimulation by humic acids (HA) has been attributed to a hormone-like effect as promoting the root development and proliferation, resulting in a more efficient water and nutrient absorption. This research aims to investigate how the humic acids isolated from vermicompost (20mg L-1) can modify the root architecture and the plasma membrane (PM) protein patterns in maize roots. It was also analyzed the effect of the citric acid (CA), an organic acid present in root exudates. The changes induced in the corn root system were estimated by measuring the taproot length, the amount of root mitotic sites and lateral roots, and the total root area. Plasma membrane vesicles were purified by cell fractionation and the protein patterns were analyzed by uni (1D) and bidimensional (2D) electrophoresis. The results show that the HA in solution with CA (0.005mM) increases the lateral root growth promotion (126%), the root area (58%), and the number of lateral roots (55%). The activity of the plasma membrane H+ pump, analyzed as a marker of the induction of the acid growth mechanism, was also enhanced (374%) by the humic solution supplemented with CA. Expression of several plasma membrane proteins was inhibited when plants were treated with HA and this effect was more pronounced upon CA supplementation. The obtained results corroborate the proposed mechanism for the HA bioactivity, by which under the action of root-exuded organic acids, such as CA, a disruption of the HA macrostructure is promoted releasing bioactive molecules presented in the humic aggregates, which becomes more accessible to the root cell receptors.
humic substances; H+-ATPases; proteomic analyzes; membrane proteins; phase partitioning
